The pin-grid array has become a popular form of integrated circuit (IC) package because it provides for a large number of pins to be associated with a plug-in package. Basically, an array of pins having a standard spacing (for example, 100 mils) is located on a printed wiring (PW) board so that they extend outward from one face for joining with the metal traces on a mother board. The pin array surrounds a central open area that houses an IC chip. The pins are contacted by metal traces on the PW board that terminate in an array that fronts on the central open area. If desired, the central area of the PW board can have a recess that is deep enough to accommodate the IC chip so that the chip bonding pads are on the same level as the array of metal traces. The IC chip is bonded in the recess and wire bonds are made between the bonding pads and the traces. Then the IC chip and its leads are covered with an encapsulant that completes the package. Alternatively, the chip pad to PW board connections can be made using a tape automated bonding (TAB) spider.
Two basic encapsulation methods have been used. In the first, a blob of plastic is cast upon the IC chip so as to cover it and the bond wires. In the second method a metal shell or lid is formed to have a skirt slightly larger than the PW board and a central raised portion that accommodates the IC chip and bond wires. This lid covers a sealing compound that fills the space inside the lid and cements it to the PW board.
Transfer molded plastic housings for IC devices have been developed to a high degree. They are mechanically stable, strong, chemically stable and, in general, compatible with IC's and their bond wires. A suitable plastic composition and the transfer molding conditions are set forth in Sporck U.S. Pat. No. 3,838,094 which is assigned to the assignee of the present invention. The teaching in this patent is incorporated herein by reference. In transfer molding the device to be encapsulated is located inside a metal mold which has recesses that define the shape of the housing to be produced. Plastic, that is ordinarily a solid, is heated and forced through gates into the mold under pressure. The heat and pressure causes it to liquify and flow into the mold cavities. The mold is heated to thermoset the plastic which thereby surrounds the IC and provides a mechanical support for the semiconductor device. The assembly can then be removed from the mold. Then the assembly can be heated to fully cure the plastic which becomes quite stable and assumes a chemical form that has the desired properties.
The basic characteristics of transfer molding are taught in U.S. Pat. No. 4,460,537 to Preston J. Heinle. The various terms used in such encapsulation are defined and the related processes detailed. The teaching in this patent is incorporated herein by reference.
It would be desirable to employ transfer molding to encapsulate an IC device located on pin-grid array using a plastic that has the desired properties.